1. Field of the Invention
This invention relates to a blow box and a method of using the blow box for inhibiting any tendency of a web to adhere to a dryer when the felt diverges relative to the dryer. More specifically, this invention relates to a blow box used in the dryer section of a papermaking machine.
2. Information Disclosure Statement
In the process of manufacturing paper, ever increasing production speeds are encountered. In order to provide support for a wet web of paper between dryer cylinders in such high-speed production of paper, it has now become customary to use a single support felt in an entire section of dryers. This felt is guided in a serpentine fashion through the two-tier dryer groups such that the felt is guided through the upper and lower dryer groups. The felt runs alternately over a top cylinder, then under a bottom cylinder and over a top cylinder of the dryer groups with the web being sandwiched between the felt and the top cylinder and being carried on the outer surface of the felt as the felt and web pass under the bottom cylinder. This arrangement is capable of providing complete support of the web through the entire dryer group provided that the web remains in contact with the felt.
However, with the advent of ever-increasing dryer speeds and particularly with speeds over 3,000 feet per minute, the web has a tendency not to stay in contact with the felt. There exist two main reasons for such tendency of the web to part from the felt. First, as the web and felt leave the top dryer cylinder, the web experiences a suction force produced by the dryer as the dryer surface separates from the web. This force tends to pull the web off of the dryer felt where the felt diverges from the top dryer. Second, the air which is entrained by the rotating bottom dryer and that portion of the felt moving between the top and bottom dryer is pumped through the felt at the converging nip. Such pumping or relatively high air pressure exists where the felt converges with the bottom dryer. Such air also tends to force the web off the dryer felt at such converging nip.
Prior proposals have attempted to control this "blowing" problem but have been directed mainly at the buildup of pressure at the converging nip.
More specifically, U.S. Pat. No. 4,516,330 to Valmet and U.S. Pat. No. 4,502,231 to Voith describer hereinafter attempt to control and reduce the pressure in the vicinity of the aforementioned converging nip.
Both of these prior proposals suffer from two primary disadvantages. First, neither of these devices address the immportance of keeping the web in tight conformity, or contact, with the felt right from the release point or diverging nip adjacent to the top dryer. Such tight contact is essential since once the web has pulled away from the felt at the diverging nip, the web has some slackness which will later allow the web to say away from the felt in response to centrifugal force as the web passes around the bottom dryer.
Furthermore, the prior proposals both teach the discharge of pressurized air into the pocket area above the bottom dryer. Such air tends to produce an over pressure in the pocket which further acts to blow the sheet off of the felt at the critical up run between the bottom dryer and a subsequent top dryer.
The present invention provides a blow box which overcomes the aforementioned disadvantages of the prior proposals and provides a blow box which makes a significant contribution to the art of web drying in a paper machine.
A primary objective of the present invention is the provision of a blow box which produces a high level of vacuum adjacent to the diverging nip of a top dryer in order to inhibit any tendency of the web to stick to the surface of the top dryer when the felt diverges relative to the top dryer.
Another objective of the present invention is the provision of a blow box having a diverging wall for inducing a partial vacuum between the box and the adjacent felt for enhancing the cohesion between the web and the felt between the top and bottom dryer.
Another object of the present invention is the provision of a blow box having a seal between the box and the bottom dryer for inhibiting the buildup of pressure at the converging nip between the felt and bottom dryer.
Another object of the present invention is the provision of a blow box which is pivotally-secured relative to the top dryer such that the blow box is able to tilt away from the top dryer in the event of the web becoming enwrapped around the top dryer.
Another object of the present invention is the provision of a blow box having a base wall which defines a opening for directing a flow or air from within the box to the pocket for inducing a partial vacuum within the pocket which tends to maintain the web in close conformity with the felt between the second and third dryers.
Another object of the present invention is the provision of a baffle which extends across the box for defining a first and second compartment, the flow of pressurized air in the first compartment inducing a partial vacuum in the second compartment such that air flows from the converging nip through a port defined by the second compartment for inhibiting any tendency of the web to part form the felt as the web passes around the second dryer.
Another object of the present invention is the provision of a first and second slot disposed adjacent to the first and second end walls of the blow box for directing a first and second current of air sideways relative to the pocket thereby avoiding the necessity for the end plate seals.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art form the detailed description, drawings and appended claims.